1. Technical Field
Since the measurement has two fundamental independent variables--pressure and temperature--both variables must be read accurately and at the proper moment in order to get reliable data. Assume temperature presents no difficultly. Pressure measurements are a problem for two reasons.
A. The zero pressure point is not known due to electronic offset, electronic drift, or atmospheric changes. In an on-line operation one must only know the range of the pressure transducer (normally 100 pounds calibrated by dead weight tester). For example, the constant or fixed offset may be +5 lbs. If the chamber is open to atmospheric pressure the computer will read and remember an offset of (+5+delta) pounds. Delta is an additional change due to variations in atmospheric pressure or electronic drift. (These two parameters are not easily separated.) Extensive work has shown that the range calibration is independent of any offset. Therefore proper zeroing (in the example zero is defined as 5+delta pounds) is necessary to get the proper pressure. The final or measurement pressure is (P-(C+delta)) where C is the constant part of the offset and delta is the variable part of the offset.
B. There is an overpressure in the flow line due to the fact that CO2 is supplied to the liquid at a pressure beyond that which can be dissolved in the beverage. By venting to the atmosphere for the same time for each type of beverage, using the third valve (equilibrium valve or timing valve), one can precisely determine the ideal time to take a measurement such that the overpressure is bled off but none of the dissolved gas has left the liquid.
TABLE 1 ______________________________________ A typical example - the comparison between a lower pressure beverage and a higher pressure beverage. LOWER PRESSURE HIGHER PRESSURE BEVERAGE BEVERAGE ______________________________________ Line pressure 30 lbs. 45 lbs. before bleed Pressure of CO2 18.8 lbs. 24 lbs. from drink after at 36.5 F. at 36.5 F. bleed and agitation Final CO2 3.34 volumes 4.05 volumes Bleed time .150 seconds .850 seconds (measurement at equilibrium solenoid valve) ______________________________________
Table shows the transition from a beverage oversaturated with CO2 in the line to a beverage with the proper amount of CO2 suitable for an on-line measurement. The calibrated leak (determined by the structure of the equilibrium solenoid valve) to the atmosphere must be timed (bleed time) to get precise and repeatable CO2 volumes that compare favorable with the laboratory backup test. The laboratory test is generally a pressure, temperature measurement similar to the on-line measurement. Different drinks required different bleed times. Some control as provided by a small computer is necessary to implement this measurement. (Note: the word sniff may be used interchangeably with the word bleed for historical reasons.)
A well-defined and repeatable bleed time is an important variable in accurately determining the pressure due only to the dissolved gas in the liquid. The sample is then agitated in the closed chamber and the resulting pressure is defined as the CO2 pressure of the drink. (Note: the computer has subtracted the zero pressure which it has in memory.)
2. Background Art
The Pressure and Temperature method is a traditional technique for deriving CO2 volumes. Most, if not all, pressure, temperature, and volume measurements are based on Heath's table (See Table 2).
The Effect of Temperature and Pressure Changes Expressed in Pounds per Square Inch and in Degrees Fahrenheit. A very useful solubility table has been calculated by Heath* in which the data are expressed in English engineering units. These data are specially useful to the bottling industry and they are here reproduced as Table 2.
TABLE 2 __________________________________________________________________________ The Solubility of Carbon Dioxide in Water at Various Temperatures in .degree.F. and Various Pressures in lbs. per sq. in. Gage. Table shows the volume of carbon dioxide measured at 32.degree. F. and 14.7 lbs./sq. in. which dissolves in one volume of water at the temperature and pressure indicated. (Calculated by Heath) P lbs./ Temperature .degree.F. sq. in. 32 36 40 44 48 55 60 65 70 75 80 85 90 __________________________________________________________________________ 15 3.46 3.19 2.93 2.70 2.50 2.20 2.02 1.86 1.71 1.58 1.84 1.35 1.27 20 4.04 3.73 3.42 3.15 2.92 2.57 2.36 2.17 2.00 1.84 1.69 1.58 1.48 25 4.58 4.27 3.92 3.61 3.35 2.04 2.69 2.48 2.29 2.10 1.93 1.80 1.70 30 5.21 4.81 4.41 4.06 3.77 3.31 3.03 2.80 2.58 2.37 2.18 2.03 1.91 35 5.80 5.35 4.91 4.52 4.19 3.69 3.37 3.11 2.86 2.63 2.42 2.26 2.13 40 6.37 5.89 5.39 4.97 4.61 4.05 3.71 3.42 3.15 2.89 2.67 2.49 2.34 45 6.95 6.43 5.88 5.43 5.03 4.43 4.06 3.74 3.44 3.16 2.91 2.72 2.56 50 7.53 6.95 6.36 5.89 5.45 4.80 4.40 4.05 3.73 3.42 3.16 2.94 2.77 55 8.11 7.48 6.86 6.34 5.87 5.17 4.74 4.37 4.02 3.69 3.40 3.17 2.99 60 8.71 8.02 7.35 6.79 6.29 5.53 5.08 4.68 4.31 3.95 3.64 3.39 3.20 70 9.86 9.09 8.33 7.70 7.13 6.27 5.76 5.30 4.89 4.49 4.14 3.86 3.63 80 11.02 10.17 9.31 8.61 7.98 7.00 6.43 5.92 5.46 5.02 4.62 4.31 4.06 90 12.18 11.25 10.30 9.52 8.82 7.74 7.11 6.54 6.04 5.55 5.12 4.77 4.49 100 13.34 12.33 11.29 10.43 9.66 8.40 7.79 7.18 6.62 6.08 5.60 5.22 4.91 __________________________________________________________________________